first step to fixing PR8642: don't fold away empty basic blocks

which have trapping constant exprs in them due to PHI nodes.
Eliminating them can cause the constant expr to be evalutated
on new paths if the input edges are critical.

llvm-svn: 122164

2 files changed, 42 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp b/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
index 60c7f7565eb..b4a2a12de03 100644
--- a/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
+++ b/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
@@ -211,6 +211,17 @@ bool CodeGenPrepare::CanMergeBlocks(const BasicBlock *BB,
   const PHINode *DestBBPN = dyn_cast<PHINode>(DestBB->begin());
   if (!DestBBPN) return true;  // no conflict.
 
+  // Walk all the PHI nodes in DestBB.  If any of the input values to the PHI
+  // are trapping constant exprs, then merging this block would introduce the
+  // possible trap into new control flow if we have any critical predecessor
+  // edges.
+  for (BasicBlock::const_iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
+    const PHINode *PN = cast<PHINode>(I);
+    if (const Constant *C =dyn_cast<Constant>(PN->getIncomingValueForBlock(BB)))
+      if (C->canTrap())
+        return false;
+  }
+  
   // Collect the preds of BB.
   SmallPtrSet<const BasicBlock*, 16> BBPreds;
   if (const PHINode *BBPN = dyn_cast<PHINode>(BB->begin())) {
diff --git a/llvm/test/Transforms/CodeGenPrepare/basic.ll b/llvm/test/Transforms/CodeGenPrepare/basic.ll
new file mode 100644
index 00000000000..04b4f34bc33
--- /dev/null
+++ b/llvm/test/Transforms/CodeGenPrepare/basic.ll
@@ -0,0 +1,31 @@
+; RUN: opt -codegenprepare %s -S | FileCheck %s
+; PR8642
+
+%0 = type <{ %1, %1 }>
+%1 = type { i8, i8, i8, i8 }
+
+@g_2 = global %0 <{ %1 { i8 1, i8 0, i8 0, i8 undef }, %1 { i8 2, i8 0, i8 0, i8 undef } }>, align 4
+@g_4 = global %1 { i8 3, i8 0, i8 0, i8 undef }, align 4
+
+; CGP shouldn't fold away the empty cond.false.i block, because the constant
+; expr that will get dropped into it could trap.
+define i16 @test1(i8** %argv, i1 %c) nounwind ssp {
+entry:
+  br i1 %c, label %cond.end.i, label %cond.false.i
+
+cond.false.i:                                     ; preds = %entry
+  br label %foo.exit
+
+cond.end.i:                                       ; preds = %entry
+  store i8* null, i8** %argv
+  br label %foo.exit
+
+foo.exit:                                         ; preds = %cond.end.i, %cond.false.i
+  %call1 = phi i16 [ trunc (i32 srem (i32 1, i32 zext (i1 icmp eq (%1* bitcast (i8* getelementptr inbounds (%0* @g_2, i64 0, i32 1, i32 0) to %1*), %1* @g_4) to i32)) to i16), %cond.false.i ], [ 1, %cond.end.i ]
+  ret i16 %call1
+  
+; CHECK: @test1
+; CHECK: cond.false.i:
+; CHECK-NEXT:  br label %foo.exit
+}
+